Nitride semiconductor element and method for manufacturing the same

ABSTRACT

A method of manufacturing a nitride semiconductor element includes dry etching a main surface of a sapphire substrate at a c-plane side thereof, using a mask provided on the main surface, to form a plurality of projections, each having a circular bottom surface; wet etching the sapphire substrate to form an upper part of each projection into a triangular pyramid shape while maintaining the circular bottom surface of the projection; and growing a semiconductor layer made of a nitride semiconductor on a dry etched surface and a wet etched surface of the sapphire substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is based on and claims priority of Japanesepatent application No. 2014-91993, filed on Apr. 25, 2014, the entirecontents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present disclosure relates to a nitride semiconductor element formedby crystal growth from a substrate, and a method for manufacturing thesame.

Description of the Related Art

A light emitting diode (LED) made of a nitride semiconductor is normallyformed by sequentially stacking an n-type semiconductor layer, an activelayer, and a p-type semiconductor layer on a sapphire substrate.Conventionally, there is proposed a means for forming projections on thesapphire substrate in order to improve the light extraction efficiency.In order to suppress deterioration of crystallinity and the occurrenceof voids during the crystal growth of a nitride semiconductor on thesapphire substrate with the projections formed thereon, the technique isproposed which involves forming an inclined surface of the projection asa crystal growth suppression surface to promote the growth of thenitride semiconductor in the lateral direction (see WO 2012/002240 A).

It is desirable that the shape of the bottom surface of the projectionhas a shape close to a regular hexagonal shape from the view point ofgood growth of the nitride semiconductor having a hexagonal crystal.However, in the technique proposed in WO 2012/002240 A, the bottomsurface of the projection is formed in a substantially triangle shape,which does not sufficiently correspond to the growth of the nitridesemiconductor, making it difficult for the nitride semiconductor to begrown in some cases. For this reason, the technique proposed in WO2012/002240 A is desired to further improve an etching method and theshape of the projection.

In a growth process of a GaN-based nitride semiconductor on a sapphiresubstrate, when crystals of the nitride semiconductor are grown, abuffer layer made of AlGaN and the like is inserted between the sapphiresubstrate and the nitride semiconductor for the purpose of therelaxation of a lattice mismatch between the sapphire substrate and thenitride semiconductor. When a sapphire substrate with projections formedthereon is used, the nitride semiconductor might be grown from a certainportion of the projection (abnormal growth) in some cases.

The nitride semiconductor is grown from the c-plane of the sapphiresubstrate ([0001] plane, or an upper surface of the sapphire substrate)not only in the vertical direction, but also in the lateral direction soas to cover the projections. The abnormal growth of the nitridesemiconductor from the certain portion of the projection can adverselyaffect the growth in the lateral direction of the nitride semiconductor.As a result, the obtained nitride semiconductor might have adeteriorated crystallinity.

SUMMARY OF THE INVENTION

Embodiments of the present invention have been made in view of theforegoing points, and it is an object of certain embodiments of thepresent invention to provide a nitride semiconductor element including anitride semiconductor with good crystallinity and a method formanufacturing the same, which promotes an appropriate growth of anitride semiconductor in a vertical direction, as well as suppresses thecrystal growth of a GaN-based nitride semiconductor from a certainportion of a projection on a sapphire substrate in order to promote theappropriate growth of the nitride semiconductor in the lateraldirection.

In view of the foregoing problems, a method for manufacturing a nitridesemiconductor element according to a first aspect of the presentinvention includes: dry etching a main surface of a sapphire substrateat a c-plane side thereof using a mask provided on the main surface toform a plurality of projections, each having a circular bottom surface;wet etching the sapphire substrate to form an upper part of theprojection into a triangular pyramid shape with the bottom surface ofthe projection kept circular; and growing a semiconductor layer made ofa nitride semiconductor on a dry etched surface and a wet etched surfaceof the sapphire substrate.

Alternatively, in view of the foregoing problems, a method formanufacturing a nitride semiconductor element according to a secondaspect of the present invention includes: preparing a sapphire substratewith a plurality of projections formed on a surface of sapphiresubstrate at a c-plane side thereof, each of the projections having acircular bottom surface and an upper part in a triangular pyramid shape,ridge lines of the triangular pyramid extending in a direction of anm-axis in a top view; and growing a semiconductor layer made of anitride semiconductor on the surface of the sapphire substrate.

In view of the foregoing problems, a nitride semiconductor elementaccording to a third aspect of the present invention includes: asapphire substrate with a plurality of projections provided on a surfacethereof at a c-plane side; and a semiconductor layer made of a nitridesemiconductor and provided on the surface of the sapphire substrate,wherein each projection has a circular bottom surface, and an upper partin a triangular pyramid shape, and each ridge line of the triangularpyramid shape extends in an m-axis direction in a top view.

According to the manufacturing method of the nitride semiconductorelement in the first and second aspects of the present invention, thebottom surface of the projection on the sapphire substrate is circular,which can easily grow the nitride semiconductor having the hexagonalcrystal, as compared to a projection with a triangular bottom surface.Furthermore, because the upper part of the projection is formed in thetriangular pyramid shape by the methods mentioned above, the inclinedsurfaces composing the triangular pyramid shape serve as a crystalgrowth suppression surface that can easily suppress the crystal growthof the nitride semiconductor, thereby suppressing the crystal growth ofthe nitride semiconductor from the projection. The nitride semiconductorelement according to the third aspect of the present invention includesa nitride semiconductor with good crystallinity, thereby improving thelight emission efficiency of the nitride semiconductor element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view showing an entire structureof a nitride semiconductor element according to one embodiment of thepresent invention.

FIGS. 2A and 2B are schematic illustrations showing plane directions ofa sapphire crystal in a sapphire substrate. FIG. 2A is an illustration aunit cell, and FIG. 2B shows a plan view of a sapphire crystalstructure.

FIGS. 3A, 3B, and 3C are schematic illustrations showing projectionsformed on the sapphire substrate for the nitride semiconductor elementaccording to the embodiment of the present invention. FIG. 3A is a planview showing arrangement of the projections, FIG. 32 is a plan viewshowing the structure of the projection, and FIG. 3C is a side viewshowing the structure of the projection.

FIGS. 4A to 4H are schematic illustrations explaining a manufacturingmethod of the substrate for the nitride semiconductor element accordingto the embodiment of the present invention. FIG. 4A is a cross-sectionalview showing a masking step, FIG. 4B is a perspective view showing themasking step, FIG. 4C is a cross-sectional view showing a midstreamprocess in a first etching step, FIG. 4D is a perspective view showingthe midstream process in the first etching step, FIG. 4E is across-sectional view showing a state of completion of dry etching in thefirst etching step, FIG. 4F is a perspective view showing the state ofcompletion of the dry etching in the first etching step, FIG. 4G is across-sectional view showing a second etching step, and FIG. 4H is aperspective view showing the second etching step.

FIGS. 5A, 5B, 5C, and 5D are schematic illustrations explaining amanufacturing method of the nitride semiconductor element according tothe one embodiment of the present invention. FIG. 5A is across-sectional view showing a buffer layer formation step, FIG. 5B is across-sectional view showing a semiconductor layer growth step, FIG. 5Cis a cross-sectional view showing an example of the nitridesemiconductor element with electrodes formed after the semiconductorlayer growth step, specifically, illustrating the cross-sectional viewtaken along the line X-X in FIG. 5D, and FIG. 5D is a plan view showingan example of the nitride semiconductor element with electrodes formedafter the semiconductor layer growth step.

FIGS. 6A and 6B are illustrations for explaining effects of theprojections formed on the sapphire substrate for the nitridesemiconductor element according to the embodiment of the presentinvention. FIG. 6A is a cross-sectional view showing the nitridesemiconductor in the process of growing in the semiconductor layergrowth step of the manufacturing method of the nitride semiconductorelement, and FIG. 6B is a partly sectional fragmentary perspective viewof the nitride semiconductor in the process of growing in thesemiconductor layer growth step of the manufacturing method of thenitride semiconductor element.

FIGS. 7A and 7B are illustrations for explaining the effects of theprojections formed in the sapphire substrate for the nitridesemiconductor element according to the embodiment of the presentinvention. FIG. 7A is a perspective view showing the projections and thenitride semiconductor in the process of growing in the semiconductorlayer growth step of the manufacturing method of the nitridesemiconductor element, and FIG. 7B is a plan view showing theprojections and the nitride semiconductor in the process of growing inthe semiconductor layer growth step of the manufacturing method of thenitride semiconductor element.

DETAILED DESCRIPTION OF THE EMBODIMENT

A nitride semiconductor element and a method for manufacturing the sameaccording to one embodiment of the present invention will be describedbelow with reference to the accompanying drawings. The drawings referredto in the description below schematically illustrate the concept of thepresent invention. In some drawings, the scale, distance, positionalrelationship and the like of respective members are exaggerated, or theillustration of a part of a member is omitted. In the description below,the same reference names and numerals denote the same or similar membersin principle, and a detailed description thereof will be omitted asappropriate.

[Structure of Nitride Semiconductor Element]

The structure of the nitride semiconductor element according to oneembodiment of the present invention will be described below withreference to FIGS. 1 to 3. As shown in FIG. 1, the nitride semiconductorelement 1 is, for example, a LED, and has a laminated structureincluding a sapphire substrate 10 as a substrate for the nitridesemiconductor element, a buffer layer 20, and a semiconductor layer 30,which are stacked on each other.

The sapphire substrate (the substrate for the nitride semiconductorelement) 10 supports the semiconductor layer 30 and is also to grow anitride semiconductor (e.g., GaN) thereon. As illustrated in FIG. 1, thesapphire substrate 10 is formed in a flat plate shape, and has aplurality of projections 11 on its upper surface. The sapphire substrate10 including the projections 11 has a thickness of, for example, 50 μmto 200 μm as a whole.

The projection 11 is to improve the light extraction efficiency of thenitride semiconductor element 1, and also to suppress the occurrence ofvoids and deterioration of the crystallinity when growing crystals ofthe nitride semiconductor on the sapphire substrate 10. Here, as shownin FIG. 2A, the sapphire substrate 10 is formed of a sapphire crystal SChaving a predetermined sapphire crystal structure, and a main surface ofthe sapphire substrate 10 consists of a c-plane of the sapphire crystal.Note that the term “c-plane” as used in the present specification mayinclude a surface slightly inclined at an inclination angle to thec-plane. The inclination angle is, for example, approximately 3° orless. The projection 11 is formed on the surface at the c-plane side ofthe substrate as the main surface. The c-plane exposed between theprojections 11 (that is, a part of the main surface without having theprojection 11) is a crystal growth surface 10 a (see FIGS. 3A and 3C).

As shown in FIGS. 2A and 2B, the sapphire crystal SC has, in addition tothe c-plane, six m-planes which are side surfaces of a hexagonal columnas shown in an illustration of a unit cell, and three a-planesrespectively perpendicular to an a₁-axis, an a₂-axis, and an a₃-axis.The direction perpendicular to them-plane is an m-axis direction. Them-axis direction includes three directions each of which extends indirection at 30 degrees to the direction of each of the a₁-axis,a₂-axis, and a₃-axis.

A plurality of the projections 11 with the same shape are arranged asshown in FIGS. 1 and 3A. As shown in the planar view of FIG. 3A, theprojections 11 are arranged in a regular manner such that distancesbetween the centers of the adjacent projections 11 are substantially thesame. For example, as shown in FIG. 3A, the projections 11 are disposedsuch that the centers of the projections are positioned at vertexes (orlattice points) of a triangular lattice. Note that the arrangement ofthe projections 11 shown in FIG. 3A is one example, and alternatively,the projections 11 may be positioned at lattice points of otherpolygonal lattices, such as a quadrangular lattice, a hexagonal latticeand the like.

A distance between adjacent projections 11 is preferably in a range of,for example, 0.2 μm to 5 μm. The width and height of the projection 11are preferably in a range of, for example, 0.2 μm to 5 μm, and 0.2 μm to2 μm, respectively. The number of the projections 11 is determinedaccording to the area of the sapphire substrate 10, taking intoconsideration the width of each projection 11 and the distance betweenthe adjacent projections 11. For example, the projections 11 are equallypositioned across the entire surface of the sapphire substrate 10.

As shown in FIGS. 3B and 3C, the projection 11 has a bottom surface 13having a circular shape, and an upper part of a triangular pyramid shapewith three ridge lines r1, r2, and r3 extending from a vertex of thetriangular pyramid. Each of the ridge lines r1, r2 and r3 extends, forexample, in the direction of each of the m-axes (see FIG. 2B) in a topview. Specifically, referring to FIGS. 3B and 3C, the projection 11includes a lower part (hereinafter referred to as a “lower projectingportion”) 111 of the projection 11 positioned at the side of the crystalgrowth surface 10 a of the sapphire substrate 10, and the upper part(hereinafter referred to as an “upper projecting portion”) 112 of theprojection 11 formed on the lower projecting portion 111.

It is necessary to shape the bottom surface 13 of the lower projectingportion 111 into a circular shape. The lower projecting portion 111 canbe formed in, for example, a truncated cone shape or a cylindricalshape, in addition to the dome shape as shown in FIGS. 3B and 3C. In allof these shapes, the side surface of the lower projecting portion 111becomes a curved surface. That is, the lower projecting portion 111means a curved part of the projection 11. The lower projecting portion111 can be formed by dry etching the sapphire substrate 10 underpredetermined conditions in the first etching step included in themanufacturing method of the substrate for the nitride semiconductorelement, which will be mentioned below.

The lower projecting portion 111 is preferably formed in a conical shapeor dome shape, rather than the cylindrical shape. The nitridesemiconductor grows from the c-place serving as the crystal growthsurface of the sapphire substrate. However, it is known that the nitridesemiconductor may be slightly grown from an a-plane perpendicular to thec-plane of the sapphire substrate. When the lower projecting portion 111has the cylindrical shape extending in the direction perpendicular tothe c-plane, the side surface of the cylindrical shape is perpendicularto the c-plane, and tangential planes of the side surface includes sixtangential planes that correspond to the a-plane. Thus, the nitridesemiconductor can be grown from the side surface of the cylindricalshape. On the other hand, when the lower projecting portion 111 has theconical shape or dome shape, tangential planes of the side surface ofthe lower projecting portion 111 is not perpendicular to the c-plane.Thus, the crystal growth of the nitride semiconductor from the sidesurface of the lower projecting portion 111 can be effectivelysuppressed.

Here, the nitride semiconductor has a hexagonal crystal. When the bottomsurface 13 of the lower projecting portion 111 is circular as mentionedabove, the following advantages are obtained.

When a nitride semiconductor is grown using a sapphire substrate withprojections on a c-plane and the projection has a surface serving as acrystal growth suppression surface (for example, projections disclosedin WO 2012/002240 A), crystals of the nitride semiconductor are firstlygrown mainly in the vertical direction from a part of the c-plane (acrystal growth surface) that does not have any projection and are alsogrown so as to cover the projections. Thus, the nitride semiconductorgrown vertically has a hole shaped due to the shape of the bottomsurface of the projecting portion. Thereafter, the nitride semiconductorgrows in the lateral direction to fill the hole therewith. In this way,a plurality of the projections on the sapphire substrate generates aplurality of holes. If the holes have different dimensions and shapes,the crystal growth of the nitride semiconductor for filling the holeswill vary among the holes. As a result, the crystallinity of the nitridesemiconductor might deteriorate.

Because the nitride semiconductor has the hexagonal crystal, holeshaving regular hexagonal cylinder shapes or regular hexagonal pyramidshapes are more likely to be generated. When the bottom surface of theprojection provided on the sapphire substrate is formed in the regularhexagonal shape, holes having the regular hexagonal cylinder shape orregular hexagonal pyramid shape reflecting the dimension and shape ofthe bottom surface of the projection can be easily formed. By formingthe projections having the bottom faces with equal-dimensioned regularhexagonal shapes, the dimensions and shapes of the holes can be setequal to each other. Thus, when the nitride semiconductor grows in thelateral direction, there hardly occurs a difference in the rates forfilling the holes and the conditions of the filled holes, which can formthe nitride semiconductors of relatively high crystallinity. In the caseof the hole having the regular hexagonal cylinder shape or regularhexagonal pyramid shape, six inner side surfaces of the hole areconsisted of the same crystal plane of the nitride semiconductors. As aresult, the crystals are grown from the respective inner side surfacesat the same crystal growth rate when the nitride semiconductor grows inthe lateral direction, which can improve the crystallinity of thenitride semiconductor.

For this reason, the regular hexagonal bottom surface of the projectionon the sapphire substrate is considered to enable production of thenitride semiconductor of high crystallinity. However, it is difficult toform the projection with the regular hexagonal bottom surface by theetching technique for use in formation of projections on a sapphiresubstrate. Here, even though the shape of the bottom surface of theprojection is not completely regular hexagonal, as long as the shape ofthe bottom surface is close to the regular hexagonal shape, andespecially a circular shape, holes with a regular hexagonal cylindershape or a regular hexagonal pyramid shape tend to be generated in thenitride semiconductor. As shown in FIG. 3B, the projection 11 with thecircular bottom surface 13 can be relatively easily formed by dryetching using a circular mask. In this way, when the sapphire substratewith the projections 11 having the circular bottom surface 13 is used toform the nitride semiconductor, holes having a substantially regularhexagonal cylinder shape or substantially hexagonal pyramid shape aregenerated in the vertical growth of the nitride semiconductor.Therefore, a nitride semiconductor with relatively good homogeneity isobtained. That is, the projection 11 having the bottom surface 13 withthe circular shape is easy to form, and can provide the nitridesemiconductor of the relatively high crystallinity. In this way, bothconditions for the manufacture and for the crystal growth can be easilysatisfied.

The term “circular shape” regarding the shape of the bottom surface 13of the projection 11 as used in the embodiments of the present inventiondoes not necessarily mean an exact circle. For example, even if theshape of the bottom surface 13 is an ellipse having a ratio of thelength of its major axis to that of its minor axis of 1.0 to 1.3, thehole having the substantially regular hexagonal cylinder shape orregular hexagonal pyramid shape can be provided when the nitridesemiconductor is grown in the vertical direction. Even if the bottomsurface 13 has a shape that is formed by cutting a part of a circle witha chord, as long as the ratio of the length of the arc to the entirelength of the circle is 0.7 or more, a hole having a substantiallyregular hexagonal cylinder shape or a substantially hexagonal pyramidshape can be provided in the same way.

The sapphire substrate 10 can be processed by dry-etching using a mask.When the mask has a circular shape, and is formed of the material thatis able to be etched by dry etching, a dome-shaped projection can beformed. When the projection has the dome shape, an angle formed by thec-plane and a tangential plane on the surface of a top part of theprojection becomes small. On the other hand, when the mask has acircular shape, and is formed of the material that is not able to beetched by dry etching, a cylindrical projection can be formed. Thecylindrical projection has an upper surface corresponding to thec-plane. Therefore, the dome shaped projection as well as thecylindrical projection can cause the growth of the nitride semiconductorfrom the top part of the dome-shaped projection and the upper surface ofthe cylindrical projection. For this reason, in the embodiment of thepresent invention, the upper projecting portion 112 of the projection 11is formed into the triangular pyramid shape by wet etching while keepingthe lower projecting portion 11 circular. Thus, the surface of the toppart of the dome-shaped projection, as well as the upper surface of thecylindrical projection are replaced by the inclined surfaces 112 aforming the triangular pyramid shape with ridge lines extending from thetip 15 in m-axis directions. Such the inclined surface 112 a is asurface from which the nitride semiconductor is hardly grown (that is,the crystal growth suppression surface), which can suppress the crystalgrowth from the upper projecting portion 112. The inclined surfaces 112a can be formed easier at the top part of the dome-shaped projectionrather than at the upper surface of the cylindrical projection.

As shown in FIGS. 3B and 3C, the upper projecting portion 112 is formedinto the triangular pyramid shape from the tip 15 of the projection 11to a predetermined position. The upper projecting portion 112 is formedcontinuously from the lower projecting portion 111. In FIGS. 3B and 3C,the upper projecting portion 112 corresponds to a part of the projection11 in the form of the triangular pyramid shape defined by the ridgelines r1, r2 and r3. The upper projecting portion 112 can be formed bywet etching the sapphire substrate 10 under predetermined conditions ina second etching step of the manufacturing method of the substrate forthe nitride semiconductor element, as will be described below.

The projection of the sapphire substrate disclosed in WO 2012/002240 Ahas an upper projecting portion including inclined surfaces and ridgelines. A position of a lower end of the inclined surface coincides witha position of a lower end of ridge line. On the other hand, in theembodiment of the present invention, as shown in FIG. 3C, the inclinedsurfaces 112 a of the upper projecting portion 112 extend below a lowerend of the ridge lines r1, r2 and r3. The inclined surfaces 112 a islarger than the inclined surface of the projection having the lower endat the same position as the lower end of the ridge line (disclosed in WO2012/002240 A). As a result, the larger inclined surface can suppressthe abnormal growth of the nitride semiconductor.

As shown in FIGS. 3B and 3C, the upper projecting portion 112 hasinclined surfaces 112 a extending in three directions. Typically, theinclined surface 112 a is similar to a surface corresponding to anR-plane of the sapphire crystal SC (with a Miller index of 10-14) (seeFIG. 2). An angle of the inclined surface 112 a with respect to thebottom surface 13 (c-plane) of the projection 11 is, for example,approximately 33° to 37°.

The range (height) of the upper projecting portion 112 is preferably 10%or more of the overall height of the projection 11, and more preferably20% or more thereof. The range of the upper projecting portion 112 maybe 50% or more of the overall height of the projection 11. The range ofthe upper projecting portion 112 is preferably 80% or less of theoverall height of the projection 11. In this way, the circular shape ofthe bottom surface 13 of the projection 11 can be sufficientlymaintained, which can enhance the effect of controlling the shape of thehole into the substantially regular hexagonal cylinder shape or regularhexagonal pyramid shape when the nitride semiconductor grows in thevertical direction. The area of the inclined surfaces 112 a of the upperprojecting portion 112 in the projection 11 can be sufficiently ensuredto effectively suppress the crystal growth of the nitride semiconductorfrom the projection 11.

When the projections 11 with the above-mentioned structure are formed onthe surface at the c-plane side of the sapphire substrate 10, crystalsof the nitride semiconductor are grown mainly from a flat surface withno projections 11 (the crystal growth surface 10 a). The nitridesemiconductor is hardly grown from the surface of the projection 11,whereby the nitride semiconductor grown from the crystal growth surface10 a is grown not only in the upward direction, but also in the lateraldirection. The hole in the nitride semiconductor formed at an upper areaof the projection 11 is filled with the nitride semiconductor grown froman inner surface of the hole toward a center of the hole to become aflat grown layer without the hole. That is, the nitride semiconductorgrown from the inner surface of the hole is joined to itself above theprojection 11. In general, when a nitride semiconductor is grown on asapphire substrate without a projection, the obtained nitridesemiconductor layer includes many dislocations extending therethroughfrom the surface of the sapphire substrate to the surface of the nitridesemiconductor layer. On the other hand, when the nitride semiconductoris grown on the sapphire substrate 10 with the projection 11, thedirection of the dislocation can be changed so as to expose thedislocation at the surface of the obtained nitride semiconductor layer.As a result, the dislocations exposed at the surface of the nitridesemiconductor layer can be reduced, and a semiconductor with highcrystallinity can be obtained. Now, a detailed description will be givenof the relationship between a crystal growth mechanism of the nitridesemiconductor and the projection 11 on the sapphire substrate 10.

When crystals of the nitride semiconductor (e.g., GaN) are grown on thesapphire substrate 10, the nitride semiconductor crystals are grown on,for example, the crystal growth surface (the c-plane of the sapphire)from which the nitride semiconductor crystals can be grown. However,since the GaN crystal is not lattice-matched to the sapphire crystal ofthe sapphire substrate 10 (that is, there is a difference in latticeconstant between the sapphire crystal of the sapphire substrate 10 andthe crystal of the nitride semiconductor), a number of dislocations aregenerated in the formed crystals due to lattice mismatch. On the otherhand, as shown in FIGS. 3B and 3C, the above-mentioned projection 11 hasthe inclined surface 112 a that is not parallel to the upper surface(c-plane) of the sapphire substrate 10.

Here, the c-plane of the sapphire substrate 10 serves as a “crystalgrowth surface” that enables the crystal growth of the nitridesemiconductor; and the inclined surface 112 a of the upper projectingportion 112 of the projection 11 has a plane orientation different fromthat of the crystal growth surface, and thus serves as a “crystal growthsuppression surface”. That is, the crystal growth from the inclinedsurface 112 a is suppressed. As explained below with reference to FIGS.7A and 7B, some nitride semiconductor 30 a tends to be grown from acertain part of the surface of the projection 11. The certain part ofthe surface is an area extending from a center of the projection 11toward the m-axis direction. The ridge lines r1 to r3 connecting theadjacent inclined surfaces 112 a (the adjacent crystal growthsuppression surfaces) can be formed at the certain part of the surfaceby wet etching, thereby suppressing the crystal growth on the inclinedsurfaces 112 a. When the projection 11 is formed on the c-plane of thesapphire substrate 10, the crystal growth suppression surface exists inthe crystal growth surface, whereby the nitride semiconductor crystal isgrown in three-dimensional growth during the crystal growth.

Dislocations are generated in the nitride semiconductor due to adifference in lattice constant between the crystal of the nitridesemiconductor and the sapphire crystal of the sapphire substrate 10.Thus, a number of dislocations are included in the nitride semiconductorvertically grown from the growth surface 10 a of the sapphire substrate10, or from the buffer layer 20 covering the growth surface 10 a.However, when the nitride semiconductor is grown in the lateraldirection to cover the crystal growth suppression surface (the inclinedsurface 112 a), new dislocations hardly occur. The crystal growth of thenitride semiconductor from the inclined surface 112 a is so slight thatthe inclined surface 112 a is kept covered with the buffer layer. Thus,when crystals of the nitride semiconductor are grown in the lateraldirection from the inner surface of the hole formed at an upper area ofthe projection 11 toward the center of the hole so as to be joined toitself above the projection 11 to fill the hole with the grown nitridesemiconductor, the crystals are grown along the surface of theprojection 11. As a result, the occurrence of a new dislocation from thesurface of the projection 11 can be prevented, and the dislocationsextending from the growth surface 10 a in the growth direction of thenitride semiconductor can converge above the projection 11, which canreduce the dislocations appearing on the surface of the nitridesemiconductor. Thus, the dislocation density of the semiconductor layer30 is reduced to thereby improve the crystallinity, so that the nitridesemiconductor element can improve its light emission efficiency.

The projection 11 on the sapphire substrate 10 has not only the effectof improving the crystallinity as mentioned above, but also an effect ofimproving the light extraction efficiency of the nitride semiconductorelement 1 because the projection 11 reflects the light propagatingwithin the nitride semiconductor element 1 in the lateral direction tocause the reflected light to propagate in the longitudinal direction.

Then, as shown in FIG. 1, the buffer layer 20 is provided to relax adifference in lattice constant between the sapphire crystal of thesapphire substrate 10 and the crystal of the nitride semiconductor grownon the sapphire substrate 10. The buffer layer 20 is formed between thesapphire substrate 10 and the semiconductor layer 30. The buffer layer20 is formed of, for example, AlN. The buffer layer 20 can be formed,for example, by sputtering under predetermined conditions in a bufferlayer formation step of the manufacturing method of the nitridesemiconductor element 1 as will be mentioned below.

The semiconductor layer 30 constitutes a light emission portion in thenitride semiconductor element 1. As shown in FIG. 1, the semiconductorlayer 30 is formed on the sapphire substrate 10 via the buffer layer 20.The semiconductor layer 30 includes a laminated structure of an n-typesemiconductor layer 31, an active layer 32, and a p-type semiconductorlayer 33 which are stacked on each other from the bottom in this order.

The semiconductor layer 30 includes GaN, AlN, or InN, or a mixed crystalthereof (In_(X)Al_(Y)Ga_(1-X-Y)N (0≦X, 0≦Y, X+Y, Y≦1)) as a group III-Vnitride semiconductor material. In the mixed crystal, a group-IIIelement may be B partially or as a whole, and N as a group-V element maybe partially replaced with P, As, or Sb.

According to the nitride semiconductor element 1 of the presentembodiment, the sapphire substrate 10 includes the projections 11 on thec-plane thereof. Each of the projections 11 has the circular bottomsurface and the upper part in the triangular pyramid shape formed by wetetching. The inclined surface 112 a of the upper part is the surfacesimilar to an R-plane (that is, the crystal growth suppression surface)which suppresses the crystal growth. When crystals of the nitridesemiconductor are grown on the sapphire substrate 10, the crystal growthof the nitride semiconductor from the inclined surface 112 a of theprojection 11 can be suppressed. As a result, the nitride semiconductorgrown in the lateral direction can be appropriately joined to itselfabove the projection 11, thereby stably exhibiting the goodcrystallinity. Note that the term “crystallinity” as used in thisembodiment means the dislocation density and the crystal orientation.According to this embodiment, the nitride semiconductor having one orboth excellent properties (that is, the low dislocation density and/orthe high crystal orientation) can be obtained. The nitride semiconductorelement 1 is formed by appropriately joining the grown crystals toitself above the projection 11, thereby improving the light emissionefficiency with little dislocation.

[Manufacturing Method of Nitride Semiconductor Element]

A method of manufacturing the nitride semiconductor element 1 accordingto the one embodiment of the present invention will be described withreference to FIGS. 4 and 5. In the manufacturing method described below,the nitride semiconductor element 1 is a LED.

First, the method of manufacturing a substrate for the nitridesemiconductor element will be described. The method of manufacturing thesubstrate for the nitride semiconductor element includes a masking stepshown in FIGS. 4A and 4B, a first etching step shown in FIGS. 4C to 4F,and a second etching step shown in FIGS. 4G and 4H, which are performedin this order. Regarding pairs of FIGS. 4A and 4B, FIGS. 4C and 4D,FIGS. 4E and 4F, and FIGS. 4G and 4H (two figures in each pair arearranged in right and left), each pair includes the cross-sectional viewat a left side and the perspective view at a right side illustration thesame step. That is, two figures in each pair illustrate the same stepfrom different points of view.

In the masking step, a mask is provided on the sapphire substrate 10.Specifically, in the masking step as show in FIGS. 4A and 4B, forexample, a SiO₂ film is deposited on the surface at the c-plane side ofthe flat-plate shaped sapphire substrate 10 with no projections 11formed thereon, and then patterned to form a plurality of circular masksM.

In the first etching step, the sapphire substrate 10 is etched.Specifically, in the first etching step as shown in FIGS. 4C to 4F, thesapphire substrate 10 with the masks M disposed thereon is dry etched toform a plurality of projections 11 on the surface at the c-plane side ofthe sapphire substrate 10. Each projection 11 has a bottom surface witha circular shape, and is formed in a conical shape or a dome shape suchas a semispherical shape as a whole. When the sapphire substrate 10 isdry etched using material for the mask M that cannot be etched, thecylindrical projection 11 will be formed by the use of a circular mask.In this embodiment, however, the material for the masks M that can beetched is used, and therefore the mask M on the sapphire substrate 10 isalso etched in the first etching step. The mask M is gradually etchednot only from its upper surface, but also from its side surface, wherebythe diameter of the mask M becomes smaller. Thus, the projection 11 onthe sapphire substrate 10 is etched in the dome shape such as thesemispherical shape, or in the conical shape. A description will begiven below of the projection that is dry etched in the conical shape ordome shape.

Specific dry etching methods suitable for use can include, for example,a gas-phase etching, a plasma etching, a reactive ion etching and thelike. Examples of an etching gas can include a Cl-based gas such as Cl₂,SiCl₄ and BCl₃; HBr; a F-based gas such as SF₆, CHF₃, C₄F₈ and CF₄; andan inert gas such as Ar, and the like.

In the second etching step, the sapphire substrate 10 is further etchedafter the first etching step. Specifically, as shown in FIGS. 4G and 4H,the second etching step involves wet etching the sapphire substrate 10on which the projections 11 with the conical shape or the dome shapesuch as the semispherical shape, are formed in the first etching step.Thus, an upper part of each projection 11 having the conical shape orthe dome shape such as the semispherical shape is formed into atriangular pyramid shape while keeping the bottom surface of theprojection 11 circular.

Note that when the projections 11 of the sapphire substrate 10 is shapedin the cylindrical shape or the truncated cone shape in the firstetching step, an upper surface thereof (the c-plane) still remains atthe end of the first etching step. The upper surface of the projections11 is etched in the second etching step into the triangular pyramidshape. When the upper surface of the projections 11 is remained at theend of the first etching step, the second etching step is performedenough to disappear the upper surface (the c plane), which may increasean etching time in the second etching step. If the etching time is toolong, the circular bottom surface may be etched into the triangularshape. Therefore, the upper surface of the projections 11 is preferablysmall, and more preferably does not exist, at the end of the firstetching step.

In the second etching step, the wet etching progresses from a tip end ofthe projection 11 in the conical shape or the dome shape such as thesemispherical shape during the wet etching process, and the upper partof the projection 11 is etched into a triangular pyramid shape.Typically, when the sapphire crystal SC is wet etched, the surface closeto the R plane is more likely to be exposed. As shown in FIG. 4H, theupper projecting portion 112 of the projection 11 is formed in thetriangular pyramid shape consisted of the inclined surface 112 a similarto the R plane of the sapphire crystal SC through the second etchingstep.

Suitable etchants for the wet etching can include, for example, aphosphoric acid, a pyrophosphoric acid, or a mixed acid produced byadding sulfuric acid to these acid, or potassium hydroxide. Conditionsfor the wet etching are preferably as follows: a temperature of theetchant of 150° C. to 300° C., and an immersion time of 1 minute to 60minutes. The conditions of the wet etching in the second etching stepare determined so that the inclined surface 112 a is exposed within adesired range.

Next, the method of manufacturing the nitride semiconductor element 1will be described. The manufacturing method of the nitride semiconductorelement 1 includes, a substrate preparing step in which the substratefor the nitride semiconductor element is prepared by the manufacturingmethod as mentioned above (see FIGS. 4A to 4H), a buffer layer formationstep shown in FIG. 5A, and a semiconductor layer growth step shown inFIG. 5B, which are performed in this order. In the manufacturing methodof the nitride semiconductor element 1, the substrate preparation stepcan be replaced with another step for preparing the substrate (that is,another substrate preparation step) in which the sapphire substrate 10with the projections 11 as shown in FIGS. 4G and 4H is previouslyprepared by another manufacturing method of the substrate rather theabove-mentioned manufacturing method (see FIGS. 4A to 4H). After theother substrate preparation step, the buffer layer formation step shownin FIG. 5A and the semiconductor layer growth step shown in FIG. 5B areperformed in this order to manufacture the nitride semiconductor element1.

In the buffer layer formation step, the buffer layer 20 is formed on thesapphire substrate 10. Specifically, in the buffer layer formation step,as shown in FIG. 5A, the buffer layer 20 is deposited on the surface ofthe sapphire substrate 10 with the projections 11 formed thereon, forexample, by sputtering.

In the semiconductor layer growth step, the semiconductor layer 30 isgrown on the surface of the sapphire substrate 10 with the projections11 formed thereon to form a light emitting element structure.Specifically, in the semiconductor layer growth step, as shown in FIG.5B, crystals of the n-type semiconductor layer 31 are grown on thec-plane side surface of the sapphire substrate 10 on which theprojections 11 formed, via the buffer layer 20. Subsequently, the activelayer 32 is grown on the n-type semiconductor layer 31, and then thep-type semiconductor layer 33 is grown thereon. In this way, the lightemitting element structure including the active layer 32 is formed. Inthe n-type semiconductor layer 31, the undoped nitride semiconductorlayer is grown without intentionally adding impurities until the nitridesemiconductor is joined to itself above the projection 11. Thereafter,n-type impurities may be added to the nitride semiconductor to grow ann-type nitride semiconductor. The semiconductor layer 30 joined toitself above the projection 11 is preferably GaN. Instead of the lightemitting element structure, another element structure such as afield-effect transistor can also be formed on the substrate 10 with theprojection 11.

Through the steps mentioned above, the nitride semiconductor element 1can be manufactured as shown in FIG. 5B. Then, a specific example inwhich the nitride semiconductor element 1 is a semiconductor lightemitting element (LED chip) is illustrated in FIGS. 5C and 5D. Thenitride semiconductor element 1A shown in FIGS. 5C and 5D includes thesapphire substrate 10 with the projections 11, the n-type semiconductorlayer 31, the active layer 32, and the p-type semiconductor layer 33which are formed over the substrate. The n-type semiconductor layer 31is partially exposed and an n-electrode 40 is provided on an exposedportion thereof. A transparent electrode 50 and a p-electrode 60 areprovided on the surface of the p-type semiconductor layer 33. Theseelectrodes can be formed in an electrode formation step after thesemiconductor layer growth step. In the electrode formation step, first,parts of the p-type semiconductor layer 33 and the active layer 32 areremoved by dry etching and the like to expose a part of the n-typesemiconductor layer 31. Then, the n-electrode 40 is formed on theexposed portion of n-type semiconductor layer 31, the transparentelectrode 50 is formed on the p-type semiconductor layer 33, and thep-electrode 60 is formed on the transparent electrode 50. As a result, anitride semiconductor element 1A shown in FIGS. 5C and 5D can bemanufactured. Note that the manufacturing method may include a dicingstep which involves dicing the light emitting element structure and thesapphire substrate 10 to divide them into individual elements after thesemiconductor layer growth step. At this time, the electrode formationstep is performed after the semiconductor layer growth step and beforethe dicing step.

In this way, according to the method of manufacturing the nitridesemiconductor element 1, the semiconductor layer is grown using thesapphire substrate 10 having the projections 11 at the c-plane sidesurface thereof, each projection 11 having the circular bottom surface,and the upper projecting portion 112 in the triangular pyramid-shapeconstructed by the inclined surface similar to the R-plane being able tosuppress the crystal growth. Therefore, the crystal growth of thenitride semiconductor from the surface of the projection 11 can besuppressed when crystals of the nitride semiconductor are grown on thesapphire substrate 10. As a result, there is substantially no abnormalgrown nitride semiconductor on the surface of the protrusion 11. Whenthe nitride semiconductor grown from the flat surface (the crystalgrowth surface 10 a) of the sapphire substrate 10 is grown in thelateral direction to cover the projection 11, it suppresses to inhibitthe crystal growth of the nitride semiconductor by the abnormal grownnitride semiconductor. Accordingly, the nitride semiconductor grown fromthe flat surface (the crystal growth surface 10 a) is more likely to beappropriately joined to itself above the projection 11, thereby stablyexhibiting the good crystallinity.

In order to confirm the effects of the sapphire substrate 10 accordingto the embodiment of the present invention in more detail, therelationship between the projection 11 and the crystal growth of thenitride semiconductor on the sapphire substrate 10 will be describedbelow with reference to FIGS. 6 and 7. FIGS. 6 and 7 illustrate thenitride semiconductor in the process of growing in the semiconductorlayer growth step of the manufacturing method of the nitridesemiconductor element 1.

Referring to FIGS. 6A and 6B, when crystals of the nitride semiconductorare grown via the buffer layer 20 on the sapphire substrate 10 with theprojections 11 formed thereon, crystals are grown not from the bufferlayer 20 formed on the surface of the projection 11, but from the bufferlayer formed over the crystal growth surface 10 a between theprojections 11. This is because the surface of the projection 11 issufficiently inclined to the c-plane of the sapphire substrate, wherebyeven in the presence of the buffer layer, a lattice constant of thecrystal of the nitride semiconductor matches worse with that of thesurface of the projection 11 than that of the crystal growth surface 10a. The nitride semiconductor has the hole H extending upward from theprojection 11. As mentioned above, the inclined surface 112 a of theupper projecting portion 112 of the projection 11 (the crystal growthsuppression surface) is the surface similar to the R-plane of thesapphire crystal SC, and the crystal growth surface 10 a is the c-planeof the sapphire crystal SC (see FIG. 2).

More specifically, as shown in FIGS. 7A and 7B, the nitridesemiconductor 30 a is slightly grown in a vicinity of a boundary betweenthe ridge lines r1, r2, and r3 of the upper projecting portion 112 and adome-shaped side surface of the lower projecting portion 111 in thesurface of the projection 11, but the nitride semiconductor is hardlygrown at surfaces of the projections 11 other than the vicinity of theboundary. Thus, the nitride semiconductor grown vertically from thecrystal growth surface (the flat surface) other than the projection 11is provided with the hole H shown in FIGS. 7A and 7B. As can be seenfrom FIG. 7A, the hole H has a shape similar to an inverted hexagonalpyramid shape expanding upward. Since the nitride semiconductor is grownin three dimensions (that is, grown vertically and laterally), thenitride semiconductor is progressively grown toward the center of thehole H when the crystal growth of the nitride semiconductor progresses.After the nitride semiconductor is sufficiently grown in the lateraldirection, the holes H are fully filled with the grown nitridesemiconductor, and the projections 11 are covered with the nitridesemiconductor, thereby forming the semiconductor layer 30 with no holeshown in FIG. 1.

The projection 11 provided on the sapphire substrate 10 in this way hasthe circular bottom surface 13, and the upper projecting portion 112 ofthe projection 11 formed in the triangular pyramid shape, therebyimproving the crystallinity of the nitride semiconductor formed on thesapphire substrate 10.

As mentioned above, the height of the upper projecting portion 112 ispreferably 10% or more of the overall height of the projection 11, morepreferably 20% or more, and for example, 50% or more. Further, in orderto keep the shape of the bottom surface of the projection 11 circular,the height of the upper projecting portion 112 in the overall height ofthe projection 11 is preferably 80% or less. The range in which theupper projecting portion 112 is formed can be adjusted by adjusting thetemperature and the immersion time of the wet etching in the etchingstep. Moreover, the range of the upper projecting portion 112 can alsobe adjusted by adjustment of the type and concentration of the etchantfor wet etching.

The nitride semiconductor element and the manufacturing method thereofaccording to the present invention have been specifically described byway of the embodiments of the present invention, but the scope of thepresent invention is not limited to the above description. It is to beunderstood that the scope of the present invention must be widelyinterpreted based on the description of claims. It is obvious that basedon the above description, various modifications and changes can be madeto these embodiments within the scope and spirit of the presentinvention.

DESCRIPTION OF REFERENCE NUMERALS

-   1, 1A: Nitride semiconductor element-   10: Sapphire substrate (substrate for nitride semiconductor element)-   10 a: Upper surface of substrate (crystal growth surface)-   11: Projection-   111: Lower projecting portion-   112: Upper projecting portion-   112 a: Inclined surface (crystal growth suppression surface)-   20: Buffer layer-   30: Semiconductor layer-   30 a: Nitride semiconductor-   31: n-type semiconductor layer-   32: Active layer-   33: p-type semiconductor layer-   40: n-electrode-   50: Transparent electrode-   60: p-electrode-   M: Mask-   SC: Sapphire crystal

What is claimed is:
 1. A method of manufacturing a nitride semiconductor element, the method comprising: dry etching a main surface of a sapphire substrate at a c-plane side thereof, with a mask provided on the main surface, thus forming a plurality of projections, each projection having a circular bottom surface; wet etching the sapphire substrate, thus forming an upper part of each projection into a triangular pyramid shape while maintaining the circular bottom surface of each projection; and growing a semiconductor layer made of a nitride semiconductor on the sapphire substrate after said dry etching and wet etching.
 2. The method according to claim 1, wherein a height of the upper part of the projection is 10% or more of an overall height of the projection.
 3. The method according to claim 1, wherein a side surface of a lower part of the projection is a curved surface formed in a conical or dome shape.
 4. The method according to claim 1, further comprising: forming a buffer layer on the sapphire substrate after said wet etching the sapphire substrate and before growing the semiconductor layer.
 5. A method of manufacturing a nitride semiconductor element, the method comprising: preparing a sapphire substrate with a plurality of projections formed on a surface of the sapphire substrate at a c-plane side thereof, each of the projections having a circular bottom surface and an upper part in a triangular pyramid shape, a ridge line of the triangular pyramid shape extending in a direction of an m-axis in a top view; and growing a semiconductor layer made of a nitride semiconductor on the surface of the sapphire substrate.
 6. The method according to claim 5, further comprising: forming a buffer layer on the surface of the sapphire substrate before growing the semiconductor layer.
 7. The method according to claim 5, wherein a height of the upper part of the projection is 10% or more of an overall height of the projection.
 8. The method according to claim 5, wherein a side surface of a lower part of the projection is a curved surface formed in a conical or dome shape.
 9. A nitride semiconductor element comprising: a sapphire substrate with a plurality of projections disposed on a surface thereof at a c-plane side; and a semiconductor layer made of a nitride semiconductor and disposed on the surface of the sapphire substrate, wherein each projection has a circular bottom surface, and an upper part in a triangular pyramid shape, and each ridge line of the triangular pyramid shape extends in a direction of an m-axis in a top view.
 10. The nitride semiconductor element according to claim 9, wherein a height of the upper part of the projection is 10% or more of an overall height of the projection. 